Heat oil in wok over high heat. Stir-fry onions in the oil until golden, 3 to 5 minutes. Transfer onions to a serving plate. Keep warm.

Add about 1/3 of the meat to wok, spreading out slices so they do not overlap. Cook slices on each side just until lightly browned, 2 to 3 minutes. Remove meat from pan and arrange over onion slices. Repeat twice to cook remaining meat.